treewide: replace builtins.toString with toString (#1657)

Link: https://github.com/nix-community/stylix/pull/1657

Reviewed-by: awwpotato <awwpotato@voidq.com>

flake/deprecation/per-system-option.nix

@@ -66,7 +66,7 @@
             ]) { inherit old new; };
             names = builtins.mapAttrs (_: lib.showAttrPath) paths // {
               until = lib.pipe until [
-                builtins.toString
+                toString
                 (builtins.match "([[:digit:]]{2})([[:digit:]]{2})")
                 (lib.concatStringsSep ".")
               ];